This invention relates to the manufacture of cold cathodes from aligned carbon nanotube (hereunder CNT) films of uniform shape that are formed in pattern on an electrode to achieve field emission of electrons of uniform intensity at low voltage. The technology of the invention is applicable to thin image displays such as a field emission display (hereunder FED).
First discovered by S. Iijima (see S. Iijima, “Helical microtubules of graphite carbon”, Nature, 354, p. 56–58 (1991)), CNTs are carbon materials in a very narrow hollow tubular form which typically range from 0.5 to 100 nm in diameter and from 1 to 100 μm in length. Today, CNTs hold promise in their application as field-emission electron sources. An electrode on which field-emission electron sources are arranged is supplied with negative voltage and emits no heat; hence, it is called a cold cathode. In particular, if CNT is used as an electron source to image displays such as FED, a multiple of CNTs must be employed since a single CNT is insufficient to provide the required emission of electrons. Furthermore, the inherent electron sources that are required to turn on the individual pixels in an FED must be insulated when the control circuit is energized.
Various methods are known in the art of producing cold cathodes capable of field emission of electrons from CNTs and they are divided into two major types, one of preparing CNTs in a separate step and depositing them on an electrode and the other of allowing CNTs to grow directly on an electrode. Methods of the first group include the following: 1) mixing CNT with conductive paste and pattering the mixture on an electrode by screen printing (see, for example, JP 11-260249 A); 2) mixing CNT with a solvent or a binder and dripping, applying or spraying the mixture to form a CNT layer on an electrode (see, for example, US 2003/0080663 A1); 3) mixing CNT with a solvent or a binder and extruding the mixture through a metal screen onto an electrode (see, for example, W. B. Choi and ten others, “Fully sealed high-brightness carbon-nanotube field-emission display”, Applied Physics Letters, 75, 20, p. 3129–3131 (1999)); and 4) passing a CNT suspension through a filter to form a CNT layer on a surface of the filter and transferring the CNT layer to an electrode (see, for example, W. A. de Heer and two others, “A Carbon Nanotube Field-Emission Electron Source”, Science, 270, p. 1179–1180 (1995)).
As a transfer method of the type described in W. A. de Heer and two others, “A Carbon Nanotube Field-Emission Electron Source”, Science, 270, p. 1179–1180 (1995), a process comprising the steps of causing an aligned CNT film to grow on a substrate and transferring it to a second substrate has been disclosed although no mention is made of a process for producing cold cathodes capable of field emission of electrons (see, for example, WO 00/73204). A method of the second group which allows CNT to grow directly on an electrode is characterized in that a catalyst deposited in predetermined positions on a surface of a substrate for electrode is subjected to CVD, thereby growing CNTs that are aligned perpendicular to the electrode (see, for example, WO 00/30141 and EP 1061554 A1).
The methods of mixing CNT with a solvent or a binder and depositing the mixture on an electrode as described in JP 11-260249 A, US 2003/0080663 A1 and W. B. Choi and ten others, “Fully sealed high-brightness carbon-nanotube field-emission display”, Applied Physics Letters, 75, 20, p. 3129–3131 (1999) are characterized in that the adhesion between electrode and CNT is sufficiently increased and also in that the adhesion provides good electrical continuity. However, CNT and other nano-scale substances have a great tendency to aggregate and are not easy to mix uniformly with other flowable substances. If an uneven mixture of CNT and other flowable substances is just deposited on an electrode, the CNT contained in the individual electron sources on the electrode does not have uniform density and in the presence of asperities on the surface of each electron source, only uneven image will be produced from the fabricated image display. With a view to mixing CNT with other flowable substances as uniformly as possible, one may increase the proportion of the solvent. However, if the solvent remains on the electrode, the field emission of electrons in high vacuum will be hampered, so the use of the solvent is desirably minimized.
In the method of transfer described in W. A. de Heer and two others, “A Carbon Nanotube Field-Emission Electron Source”, Science, 270, p. 1179–1180 (1995), no binder is used and the solvent is removed by filtration. However, the CNT film on the filter is immediately deposited on an Teflon sheet as the electrode, so this method is not suitable for patterning purposes. Another problem concerns the adhesion between electrode and CNT.
In order to fabricate a cold cathode that is capable of field emission of electrons and which is suitable for use on FED, the aligned CNT film must be patterned in blocks and the control circuit be energized with individual blocks being kept insulated. However, WO 00/73204 does not disclose any method of forming a pattern of aligned CNT film on the surface of a substrate for electrode.
As taught in WO 00/30141 and EP 1061554 A1, CVD may be applied to a substrate for electrode so as to form a pattern of aligned CNT film. However, the substrate for electrode employed in those methods is exposed to high-temperature carbon precipitating conditions which may potentially deteriorate the material of the substance.
In order to operate an image display using a cold cathode capable of field emission of electrons, it is advantageous that electrons of uniform intensity are emitted at the lowest possible voltage. To this end, a multiple of CNT electron sources to be used in the cold cathode capable of field emission of electrons are preferably of such a shape that the individual CNTs form unit films in which they are aligned vertically to the electrode at a specified height and that the individual films are electrically insulated from each other. If the vertical alignment requirement is met, the multiple of CNT electron sources put together can provide a maximum sum intensity of electron emission in vertical direction. If the individual films are uniform in height to present smooth even surfaces, electron emission that can be obtained is uniform in planar direction. In the case of field emission of electrons, the smaller the distance between the tip of each CNT and the anode, the lower the voltage that is required to extract electrons. Hence, given a constant height of electrode sources in each unit film, the anode can be installed very close to the surfaces of the electron sources without compromising the uniformity of distance and the applied voltage for obtaining the same intensity of electron emission can be lowered.